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This song goes Fm9 - Bb13 - Bbm9 - C7b9(#5) As far as I can tell this is a 1-4-5 in F minor... but what about the Bb13 ? Note that this vamps repeats over and over ...the song is pretty fast ..maybe 2 seconds on each chord maximum... Should it be considered just a passing chord ? What is the theory behind ? Thanks!

Wow , this generated more discussion than I thought ....here is the song :

...I`m analysing all the answers you folks kindly shared... Thank you !!

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    I don't understand the downvote. This is a perfectly valid question about harmonic theory. – jdjazz Jun 11 '18 at 5:12
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    The downvote(s) could easily be because we aren't given any opportunity to listen to the piece in question - for all we know, the chord names may even be inaccurate! Be nice to know though. – Tim Jun 11 '18 at 15:02
  • Chordify has an interesting (and inaccurate) take on the chords... – Tim Jun 12 '18 at 8:44
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For starters, this is definitely an F minor progression. Do not interpret this as Eb in the absence of any Eb chords.

IV7 while in a minor key is a common use of modal interchange. It's borrowed from the dorian mode. It's common because it's a very strong suggestion of dorian (since it contains the nat6) and it is easy to return to the original key (by flatting the sixth again). This also creates a chromatic voice leading starting from the b7th of the scale and descending to the 5th if you then go to V7 or the root (in this case, Eb-D-Db-C).

By extension, it is useful to interpret the BbMi9 as the restatement of the key. You often see Db7#11 in this position, and in that sense it is somewhat unusual to see the ninth (C), but there is no need to overthink this. BbMi9 represents the subdominant of the key and the progression is a typical minor i-iv-V with a quick trip to the parallel dorian.

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It's a kind of modal borrowing--in the context of F minor, Bb13 is arguably borrowed from the tonic major (F major). Think of it as IV13.

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    B♭13 isn't diatonic to Fmaj, so how can the chord B♭13 be borrowed from Fmaj? – jdjazz Jun 11 '18 at 5:29
  • It's more of an incomplete borrowing. Given that the other answers often skip the added tones in the chords (Laurence Payne saying the 2nd chord is V7 instead of V13, your answer saying the 1st chord is Fm6 instead of Fm9), I figured the added tones don't quite count harmonic function-wise and it's the root, third, and fifth that really matter. – Dekkadeci Jun 11 '18 at 13:55
  • Fm6 & Fm9 are equivalent in way that B♭M7 and B♭7 are not. The reason I emphasized Fm6 is just to point out that the i chord can be thought of as containing a natural 13th. I could change it from Fm6 to Fm69 and my answer would still stand. Adding or removing the 6th or 9th does not change the chord quality. By contrast, removing the flat 7th from B♭13 does change the chord quality--significantly. Similarly, in Laurence's answer, adding or removing the 13th doesn't change the chord quality. He's just simplifying, which is great because then his answer applies to any V7 chord (V9, V13, etc.). – jdjazz Jun 11 '18 at 14:09
  • @jdjazz Considering Bb13 contains the notes Bb D F G, how can it not be diatonic to F major? – coconochao Jun 11 '18 at 18:25
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    @coconochao, the chord you spelled is Bb6, not Bb13. The chord Bb13 implies a flatted 7th (an Ab), whereas the Bb6 doesn't imply any 7th. For more on the conventions of chord symbols, check out this outstanding answer by Dom, one of the moderators: music.stackexchange.com/a/20561/40842 – jdjazz Jun 11 '18 at 19:34
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Overview

There is a good way to interpret the progression so that the B♭13 isn't out of place/being borrowed from a different mode. Think of the first two measures as being in F Dorian and the last two measures as being a iiø-V7alt in F min. The advantage of this approach is that there is more cohesion/continuity between the Fmin and B♭13 chords.

Think of B♭min as Gø7

The B♭min chord is serving a subdominant function: it preps the V7 chord, C7alt. In this way, it is interchangeable with Gø7. In fact, if we replaced B♭m9 with B♭m6 (which wouldn't change the function of the B♭m chord), then now the B♭m6 chord would be voiced B♭-D♭-F-G, which is also the exact spelling of a Gø7 chord. In general, ii-V-i progressions and iv-V-i progressions are both commonly found in music, and are often interchangeable.

Think of Fm9 in terms of F Dorian

Just as we could treat the B♭m9 chord as B♭m6, the Fm chord can be interpreted through many different scales (F melodic minor, F Dorian minor, F Aeolian minor, etc.). If you want the first two measures to be continuous, then you would choose F Dorian minor for measure 1, since F Dorian contains the D♮ from the B♭13 chord. In effect, you'd be treating the Fmin9 chord as Fmin6 (or more technically Fmin6/9). This interpretation/approach would create good cohesion in the first two measures, since both chords would now contain a D♮. When improvising, for example, an Fmin6 pentatonic scale would sound great (F-G-A♭-C-D) and would be seamless when passing from one chord to the next.

So ultimately, the way to think about this progression is as a i-IV7-iiø7-V7alt, with the first two measures using F Dorian and the last two measures being a turnaround:

| Fm6/9 | B♭13 | Gø7 | C7alt |

Side Note

Other interpretations are possible. For example, you could interpret the Fm9 chord as implying F Aeolian (i.e., F natural minor). Doing this would cause the B♭7 chord to stick out more, because it'll be the only chord in the four measures that contains a D♮. One downside of using F Aeolian is that the D♭ will be an avoid note over the Fm9 chord, whereas the D♮ wouldn't. If you use F Dorian, then now the D♭ from C7alt can resolve down a half step to C or up a half step to D♮.

So if you're trying to make the B♭7 chord be more cohesive with the other chords, I recommend the approach I've outlined above. (Of course, if you're comping/playing chords behind a soloist, you want to listen to what the soloist is playing and let that dictate your choices too. If the soloist is playing a D♭ over the Fm9 chord, then avoid a D♮.)

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Seems it might be using F Dorian, Which sounds similar to F minor, but the IV chord becomes Bb major. The fact that it's a 13th chord, with at least a 6th and a m7th doesn't really make a lot of difference. Then maybe it reverts to Fm proper, before going to V on C. The Bb major could be called a borrowed chord from the parallel key of F major, if you want a bit of alternate theory.

Having listened to 'this song', it might make matters different...

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Try hearing it as ii7, V7, v7, VI7 in Eb. The fact that it isn't wrapped up with Fm7, Bb7, Eb doesn't matter.

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    I know for fact the song is in F minor. Even the composor said (it`s sort of a blues) The question still stands... – chips Jun 11 '18 at 0:16
  • Thinking of it in E♭ would make sense if it were being used as a turnaround (basically like a ii-V7-iii-VI7 progression). Given that it's a vamp which never resolves to E♭, I think it makes more sense to think of Fm as the tonic, as the composer and poster has said. That way, the progression starts with the i chord and ends with the V7 chord. – jdjazz Jun 11 '18 at 5:35
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    It isn't in in Eb at all, I don't see how thinking like this could help. – coconochao Jun 11 '18 at 18:16
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    It's in Fm, or more clearly, F Dorian - which is the Dorian of Eb, which will contain the same basic notes. However, since the chords have several extensions, thinking in Eb doesn't help much. – Tim Jun 12 '18 at 8:37
  • It seems we agree, F Dorian is a re-rooted Eb major. If you're looking for a functional analysis, it might be better to avoid the modal description. – Laurence Payne Jun 12 '18 at 9:48

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